A conventional MOSFET includes a semiconductor wafer having source, body and drain regions of alternate conductivity type disposed in series therein. A portion of the body region is located at a surface of the wafer and is bounded by a source/body PN junction and a body/drain PN junction so as to define a channel region in the body region adjacent to the wafer surface. An insulated gate electrode overlies this channel region on the wafer surface.
When a voltage greater than some threshold voltage is applied to the gate electrode, it produces an inversion in the conductivity type of the body region in that portion of the channel region that is contiguous with the wafer surface. The so-called inversion channel thereby produced permits a unipolar current flow between the source and drain regions. This unipolar electron flow (for N channel devices) or hole flow (for P channel devices) is selectively modulated by the voltage applied to the gate. However, this source/body/drain structure inherently also produces a parasitic NPN or PNP bipolar transistor which is detrimental to FET performance.
In a conventional MOSFET structure the body region is formed by diffusion (or implantation and diffusion) of an appropriate dopant through an aperture in a mask which is disposed on the wafer surface. The mask aperture is typically of a polygonal shape, such as square or hexagonal, and the mask may include a plurality of apertures, so as to form a plurality of body regions. In what is commonly referred to as a double diffused MOSFET, the source region is formed by diffusing an appropriate dopant of opposite conductivity type through the same mask aperture(s).
In a double diffused structure, the differential diffusion rate between the body region dopant and the source region dopant creates a spacing between the source/body PN junction and body/drain PN junction for each body region. At the semiconductor surface, this spacing defines the channel region in each body region. The length of the channel region is defined as the distance between the source/body PN junction and body/drain PN junction at a particular point, and might typically be in the range of a fraction of a micron to several microns. The width of the channel region is measured perpendicularly to its length.
However, this conventional polygonal channel region geometry intrinsically creates certain problems. The gain of the parasitic bipolar transistor and the threshold voltage of the MOSFET is nonuniform at different portions of the channel region. This occurs because in the corners of the polygonal channel regions there is a different dopant concentration profile compared to that along the edges of the polygonal channel regions.